Oil well logging has been known for many years and provides an oil and gas well driller with information about the particular earth formation being drilled. In conventional oil well logging, after a well has been drilled, a probe known as a sonde is lowered into the borehole and used to determine some characteristic of the formations which the well has traversed. The probe is typically a hermetically sealed steel cylinder which hangs at the end of a long cable which gives mechanical support to the sonde and provides power to the instrumentation inside the sonde. The cable also provides communication channels for sending information up to the surface. It thus becomes possible to measure some parameter of the earth's formations as a function of depth, that is, while the sonde is being pulled uphole. Such “wireline” measurements are normally done in real time (however, these measurements are taken long after the actual drilling has taken place).
Porosity measurements are commonly done by using a dual detector neutron logging tool using a source of neutrons irradiating the formation being studied. Density measurements are commonly done by using a dual detector gamma ray logging tool using a source of gamma rays irradiating the formation being studied. The density measurements, and some of the porosity measurements, may require the use of a radioactive source of neutrons and/or gamma rays. From a safety standpoint, the use of radioactive sources is problematic, particularly for measurement while drilling (MWD) applications.
Radioactive-source-free tools, such as Nuclear Magnetic Resonance (NMR) and acoustic logging have been used in the past for porosity determination. NMR logging has the advantage of directly measuring fluids in pore space and, thus, does not suffer from the lithology effect on porosity determination. However, the accuracy of NMR total porosity in gas-bearing formations is affected by low hydrogen index (HI) and the ability to separate gas and liquid NMR responses. In the cases of extremely viscous oil-bearing reservoirs, coal-bed methane-bearing formation or gas-hydrates, the hydrocarbon signals may relax too fast to be observed by the state of art NMR well logging instruments, thereby causing under-estimation of porosity. On the other hand, acoustic measurements respond to lithology and texture in addition to porosity. Consequently acoustic porosity is an indirect measurement based on empirical or semi-empirical models, which often requires calibration of model parameters.
Integrating acoustic and NMR measurements for gas-zone porosity estimation has been reported in relatively clean sandstones. However, the existing methods in literature have not been extended to shaly sandstones. The present disclosure describes a radioactive source-free porosity estimation method using NMR logs to calibrate acoustic porosity models. This approach is applicable to clean and shaly sandstones using wireline and logging while drilling (LWD) measurements.